Wood inspection by infrared thermography1

A. Wyckhuyse, X. Maldague Corresponding author

Electrical And Computing Engineering Department, Université Laval - Quebec City (Quebec) Canada GIK7P4,

Ph: (418) 656-2962, fx: (418) 656-3594, email: maldagx@gel.ulaval.ca

Abstract

Wood is used everywhere and for everything. With times, this material presents many adulterations, witch degrade his physical properties. This work present a study of infrared thermography NDT for wood decay detection. The study is based on the difference of moisture content between sound wood and decay. In the first part, moisture content influence on response signal is determine. The second part define the limits of infrared thermography for wood decay detection. Results show that this method could be used, but with many cautions on depth and size of wood defects.

Keywords

Wood inspection, infrared thermography, non-destructing testing

1. Introduction

All over the world wood is a universal, easy to process material used for important infrastructures, but this material present with times, many degradations. Moisture and temperature variations affected by local conditions are the principal factors affecting the rate of wood decay. Obviously, wood deteriorates more rapidly in warm, humid regions with respect to cool or dry places. Principal organisms degrading wood are fungi, insects, bacteria and marine borers with damages ranging from discoloration to complete wear of wood with possible catastrophic consequences such as total breakdown of involved structures.

To prevent and control those effects, keeping costs down, a non-destructive measuring of wood conditions and so mechanical properties is needed.

This presentation report work based on infrared thermography to detect wood decay. This work is based on the thermal stimulation of wood with observation during subsequent cooling. The basic principle uses the fact wet rotten (with 30 % moisture content corresponds to the low-limit of free water) and sound wood have different thermal properties so that inspection of good and rotten specimens would bring temperature differences proportional to the moisture content and so to the decay level. This work is based on the thermal stimulation of wood with observation during subsequent cooling. The approach is generalized to all types of woods and defects, the aim being to present IRT applications for nondestructive detection of damages in wood.

1 From : Proc. of IVth. IWASPNDE, X. Maldague ed., TONES (ASNT pub.), 6: 201-206, 2002.

2. Preliminary study

The first step consists in determining the influence of moisture content on the difference signal, at different depths. This study contained a theoretical part and experiments to compare both. Heating is done to upper surface (front) and measurement are performed in reflection and transmission.

Modelling results are following, for a rotten sample, that is a wood specimen fully impregnated with 21, 30 and 40 % of moisture content figure 1 (measurement performed in reflection), and for a wood specimen with a subsurface defect located at three different depths figure 2; heating and measurement were performed in reflection.

Results indicate that the difference of up to 20 °C is observed for moisture content of 40 %. But, this temperature signal decrease with moisture content, and very quickly with depth. At two centimetres below surface, the signal decreased so much, that the difference of temperature is no longer practically measurable.

Figure 1 : Rotten wood with different moisture content. Heating power of 2500 W during 550 s

In transmission mode, the results shown confirm what was observed in reflection in figure 2: the signal can be measured only for a limited depth or after a long time. At depth greater than 6 cm, it is needed to wait more than 2000 s to obtain a difference of slightly more than one degree. Transmission is thus not particularly suitable when a fast inspection is required and obviously also when access is restricted to the front surface.

Figure 2 : Wood with subsurface defect located at different depth. Heating power of 2500 W during 550 s

Having established the range of possible detection depths, the study of wood with more realistic defects can now take place.

3. Defects detection

3.1 Theoretical study

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Moisture content = 21% Moisture content = 30% Moisture content = 40%

Decay at 1 cm depth Decay at 1.5 cm depth Decay at 2 cm depth

A cubic sample geometry is selected being common in industrial products. Same values for thermal parameters and moisture content are kept. The two measurement modes are tested (reflection and transmission) with heating applied on the front surface.

Figure 3: Geometry used for theoretical study of reflection and transmission modes

The three considered geometries are illustrated on figure 3. In figure 3-a, defects are of circular shapes and small size (0.5, 1, 1.5 and 2 cm at 0.5 cm depth), in figure 3-b, defects diameter is same (2 cm) but at different depth (0.5, 1 and 1.5 cm), while geometry of figure 3-c is created to study larger defects (10 cm length and 1 cm width at 1, 1.5 and 2 cm depth) since, in practical situations, defects have to be large enough in order to significatively reduce wood resistance and thus to be interesting for detection. Modelling results are next presented for each case of figure 3.

Reflection measurement mode

Transmission measurement mode

Figure 4. Theoretical modeling in reflection and transmission geometry of figure 3

Results of figure 4 for geometries of figure 3 confirm what was obtained precedently. In reflection, for depth greater than two centimeters, the signal is very low and often take a long time to develop. Figure 3-c exhibit more stronger signals due to bigger-size defects, especially for the 1 cm deep flaw. Despite these, the signal remains low and important depth detection can not be expected.

For the transmission mode, as it is well known, it is not possible to establish defects depth because whatever is the depth, the signal arrives at the same time. This is confirmed on figure 6. Moreover the signals are very low and in fact undetectable in practical situations but, perhaps in the case of figure 4-c.

3.2 Experiments

3.2.1 Reflection measurement

In order to confirm modeling results, experiments were performed with specimens of similar shapes with respect to simulations presented before. However, due to the difficulty to obtain uniform heating over a large surface, lateral extension of specimens is reduced. Subsurface defects are located as shown on figure 3, although 10 cm in length instead of 1 meter. In order to reproduce internal defects, a wood piece is cut in half, one half remained intact and holes were drilled in the other half with same diameters than for theoretical investigation (sample of figure 3-c is not tested due to its large size).

Samples with dry defects

Samples with wet defects

Figure 5. Experiments in reflection on samples with dry and wet defects, geometry of figure 3

During experiments both halves are joined together to form a single specimen with one sound and one defective part. This is found particularly attractive to compare defect and no-defect responses. After the heating period, images are recorded into the computer and the temperature map of each image is computed with a Matlab program. A Cincinnati Electronics Focal Plane Array (FPA) type of infrared (IR)

a a

b b

b b a a

camera operating in the 2-5 µm is used. Heating is done during 30 s with lamps (power 7200 W). Results are as follow.

Dry defects are first studied. They are made up by filling holes with a dry material (wood chips) of same humidity content as dry wood, that is 15 %. Images of figure 5 show that dry defects are not detected. Nevertheless wood surface structure is clearly distinguished enabling surface control. Images of wet defects reveal some of the subsurface holes ( figure 5). Defects are made up by filling holes with a wet material such as humidified paper towel. This corresponds somehow (but not exactly) to the hypothesis of the modeling that assumed a 30 % level of humidity. In the first case (geometry of figure 3-a), the defect nearest the surface is detected, the second defect appears a little after 250 s, but the last one is never observed. On the second case (geometry of figure 3-b), only the shallower defect is detected, the others not. As theory indicated, only large or close to the surface defects are detectable, others could only be detected provided higher power heating is available, but this would however damage the wood (see Part I). Theory indicated a difference of temperature of about 3 °C is to be expected for large, close to the surface defects while for small, deeper defects, the difference reduces 1°C. Such values are somehow confirmed in practice but with a poor defect definition due to the fact holes were drilled side by side.

Another experiment was performed using microwave heating. Microwave heating is explored to study response to global heating. Sample tested on Figures 5 (geometry of figure 3) is investigated for these experiments. The specimen is introduced in a domestic microwave oven (1250 W). After two minutes of heating, the microwave door is opened and the IR camera pointed to the specimen for thermograms recording. As in the last section, dry and wet defects are tested, figure 6 presents the results.

Figure 6. Experiments in reflection microwave heating for 2 min, geometry of figure 3

With dry defects nothing is detected since differences of thermal properties between sound wood and holes filled with wood chips of same humidity content is not significative. Wet cases ( figure 6-a & b) are more interesting with largest defects detected and for figure 6-a, defect geometry visible. Defects of figure 6-b appear with some blur since they are deeper with respect to those of figure 6-a. Presence of smaller defects appear less clearly (enlarged blob at 200s). As before, only large, close to the surface wet defects can be detected.

3.2.2 Transmission measurement

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In the transmission approach, theory previously developed indicated expected temperature difference should be about 0.1°C for specimen geometry of figure 3, a signal difference barely visible in practice. In fact, experiments performed in transmission with the previous sample revealed nothing significative. Nevertheless, in order to investigate the transmission approach, another sample is prepared with shallower defects. Figure 7 presents a picture of this sample as well as thermograms and phase images. Heating is made on the face with holes, and measurement on the other face. All defects are detected, the deepest ( in the centre) is especially visible on the phase picture. The depth of this last defect is 8 mm, the temperature difference is less than one degree.

Figure 7. Experiments in transmission on samples with drilled holes.

This confirms previous discussions about the transmission approach which is not further investigated.

IV CONCLUSION

This study pointed out interesting facts. First of all, IRT can be used for wood inspection but with some cautions due to any adverse variables such as unknown wood moisture content and very low thermal diffusivity. The starting hypothesis of this study was that rotten wood defects have a different moisture content with respect to sound wood. In fact theory confirmed by experiments showed that dry defects could not be detected by IRT, thus confirming the original hypothesis. Obviously this however limits IRT application for rotten wood investigation. Nevertheless subsurface defect detection is possible as demonstrated in this paper although quantitative investigation is difficult. Even if geometry of subsurface defects can not be obtained accurately due to very low wood thermal diffusivity, it is still possible to get cues about subsurface defect depth or dimension. IRT can be used for small defects at small depths. Bigger flaws at larger depths can be detected but after a longer waiting period. IRT present many advantages for defects detection in wood, especially for investigation at small depths.

V ACKNOWLEDGMENTS

The supports of the Natural Sciences and Engineering Research Council of Canada and of the Fonds FCAR from Québec Province are acknowledged.

REFERENCES

X. Maldague, Theory and Practice of Infrared Technology for Non Destructive Testing, John-Wiley & Sons, 2001.

A. Wyckhuyse, X. Maldague, “A study of wood inspection by infrared thermography, Part I: Wood Pole Inspection by Infrared Thermography,” Research in NonDestructive Evaluation, 13 [1]: 1-12, March 2001.

A. Wyckhuyse, X. Maldague, “A study of wood inspection by infrared thermography, Part II: Thermography for Wood Defects Detection,” Research in NonDestructive Evaluation, 13 [1]: 13-21, March 2001.

Face seen Heating Face

Time 90 s Time 120 s PHASE 4